For example, European Published Patent Application No. 0 477 418 describes an ultrasonic flow rate meter which is suitable for installation in a measuring pipe through which a fluid flows, in particular for measuring air mass in a motor vehicle engine. Two ultrasonic transducers operate alternatingly as transmitting or receiving transducers; however, the emitted ultrasound waves travel via a reflector system along a measuring path through the measuring pipe between the ultrasonic transducers. The individual modules are preferably integrated with an operating and measuring electronic unit on a circuit board as a built-in unit which may be installed sealed in a receptacle opening of the measuring pipe.